Treatment of titanium metal surfaces



United States Patent TREATNEENT F TITANIUM METAL SURFACES SvanteMellgren, Laurence Harbor, and Oliver W. Moles,

No Drawing. Application September 17, 1952, Serial No. 310,132

7 Claims. (Cl. 134-30) This invention relates to a method for picklingrefractory metals. More specifically, it relates to a method forremoving scale such as oxide coatings and the like from a titanium metalsurrace.

'l'itanium metal particularly at elevated temperatures has a strongamnity for oxygen, nitrogen, hydrogen and the like, and contact betweenthe metal and these gases, boil]. in the initial production steps andalso in subsequent fabrication steps, orten results in the formation ofsurface scale or coatings of the oxide, nitride, etc. when this typescale or coatin is formed on such titanium metal products, thepropertiesare often modlned to such a degree that the roducts are nolonger suitable for the purpose for which they had been intended.rurtnermore, such surrace scale adheres very tenaclously to the titaniummetal and employment of pickling processes such as, for example, theacidic washes customarily used tor the removal of scale rrolll iron andsteel have proved ineffective.

An OOJCCE or this invention, therefore, is to provide a satlslactoryprocess tor removing such surrace scale or coating lronl titanium metalin order to make the metal suitable tor commercial utilization. Anotherob ect is to provide a quick and emclent method for reclaiming titaniumrrom titanium metal products which are so contamlnated. l hese and otheroo ects will become obvious from the following more complete descriptionof the instant invention.

.Dl'OklQly speaking the instant invention contemplates a process torremoving scale from a titanium metal surrace which comprises treatingsaid surface with a molten alkali metal polysulnde until said scale 18loosened trom said surface and removing said polysulfide and saidloosened scale rrom said surtace.

Alkali metal polysulfides which are suitable for employment m theinstant process may be produced in a number or ways. rot example, analkali sulfide such as sodium sulfide tNaaS) may oe combined withadditional free sull'ur to yield compositions corresponding to thegeneral formula NazSa wnere x is greater than unity. A particularlyeffective composition comprises either sodium or potassium polysulfidesin which from about 0.1 to parts of free sultur are present in additionto that normally combined as NazS or ms. Mixturesof sodium and potassiumpolysulfides may also be used.

The polysultide composition must be in the molten state. lhetemperatures at which such compositions melt vary considerably and is inpart dependent upon the amount of free sulfur present. Satisfactoryscale removal has been accomplished at temperatures or from about 250 C.to 500 C., naturally depending upon such factors as bath composition,apparatus size, design, etc.

When the titanium metal surface is treated with the molten polysulfide,the surface scale is loosened very rapidly, and generally, the metalproduct need be exposed to the action or' the polysulfide for only arelatively short period of time. According to the instant process thetreatment time should not exceed about ten minutes. Obviously shorterperiods or contact may also be employed dependent upon the amount ofscale and type of surrace to be treated. However, it may be desirable onoccasion to extend the treatment time beyond the ten minute periodindicated above. For instance, the size of the article or the method ofapplication of the polysulfide may be such that a longer time periodwould be more satisfactory. Accordingly, it has been found possible tobuffer the action of the molten polysulfide by the addition of halidecompounds such as, for example, the alkali and alkaline earth ice metalhalides including magnesium halides. These compounds n general aremiscible with the polysulfides employcd in the instant process.Obviously, the amount of halide compound which is used may vary and willdepend on the degree of bufiering action which is desired and upon thetemperature to be employed. For practical utilization of such bufl'eredpolysulfide melts it has been found convenient to limit the amount ofhalide to that amount which will permit formation of a moltenpolysulfide-halide mixture at temperatures of from 250 Cv to about 500C. Furthermore, the addition of the buffer compound generally should belimited so as to permit the treatment time to be extended up to aboutone hour since longer periods are not particularly desirable incommercial operations.

In a further embodiment of the instant invention it may be desirable torinse the metal surface from which the scale has been loosened withwater to remove any remaining traces of loosened scale and polysulfidewhich may be present. The surface of the metal then possesses a fresh,clean metallic luster.

The actual method of treating the titanium metal surface with the moltenpolysulfide may, of cource, vary; however, according to one preferredmeans, a molten bath of polysulfide is provided in which the titaniummetal article which contains the surface scale may be appropriatelyimmersed for a time sufiicient to loosen said sca e.

in order to describe the instant invention more clearly, the followingexamples are presented:

Example I A piece of titanium bar stock, which had been hot rolled in anoxygen atmosphere and had been contami-- Example II A titanium metalingot which was covered with a surface scale of titanium oxide andtitanium nitride was immersed in a molten'bath comprising a mixture ofsodium polysulfide and potassium polysultide, the composition of whichcorresponded closely to the formula NamrsKzS. The ingot was permitted toremain in contact with the molten polysulfide mixture for a period offive minutes at a temperature of about 500 C. The ingot was thenwithdrawn from the polysulfide melt and subsequently cooled. Again, thesurface scale had been completely reinoved, and the ingot possessed abright and shiny metallic uster.

By the examples presented and by the preceding discussion, it has beendemonstrated that according to the process of the instant invention, itis possible to effectively and swirtly remove surface scale such asoxide coatings from titanium metal by treating the metal surface withalkali metal polysulfide. This quick and efficient operation permits thereclamation of large quantities of titanium metal which would otherwisenot be readily available for commercial utilization.

While this invention has been described and illustrated by the examplesshown, it is not intended to be strictly limited thereto, and othervariations and modifications may be employed within the scope of thefollowing claims.

We claim:

1. A process for removing adherent scale from a titanium metal surfacewhich comprises treating said surface with a molten alkali metalpolysulfide until said scale is loosened from said surface and removingsaid polysulfide and said loosened scale from said surface.

2. A process for removing adherent scale from a titanium metal surfacewhich comprises treating said surface with a molten alkali metalpolysulfide for a period of not more than about ten minutes to loosensaid scale from said surface and thereafter removing said polysulfide.and

said loosened scale from said surface.

3. A process for removing adherent scale from the surface of a titaniummetal product which comprises immersing said product in a molten alkalipolysulfide for a period of not more than about ten minutes until saidscale is loosened from said surface, substantially immediatelythereafter Withdrawing said product from said molten polysulfide, andrinsing With Water to remove anyremaining traces of loosened scale andpolysulfide.

4. A process according to claim 1 wherein said polysulfide comprisessodium polysulfide.

5. A process according to claim .1 .Wherein'sa'idpo'lysulfide comprisespotassium polysulfide.

6. A process for removing adherent scale from a titanium metal surfacewhich comprises treating said surface with a molten alkali metalpolysulfide and a compound selected from the group consistingof alkalimetal halides and alkaline earth metal halides including magnesiumhalide .until said scale .isloosened from said surface and removing saidpolysulfide, said compound and said loosened scale from said suit-face.

7. A process for removing adherent scale from a titanium metal surfacewhich comprises treating said surface with a molten alkali metalpolysulfide and a compound selectedrfrorn the group consisting of alkalimetal halides and alkaline earth metal halides including magnesiumhalide for a period of not more than about one hour to loosen said scalefrom said surface andthereafter removing said polysulfide, said compoundand said loosened scale from said surface.

References Cited :in the file of this patent The Canadian Mining andMetallurgical Bulletin for January 1951, Montreal, pages 2'528(photostat in Div. 3) Ti Digest.

1. A PROCESS FOR REMOVING ADHERENT SCALE FROM A TITANIUM METAL SURFACEWHICH COMPRISES TREATING SAID SURFACE WITH A MOLTEN ALKALI METALPOLYSULFIDE UNTIL SAID SCALE IS LOSSEDEN FROM SAID SURFACE AND REMOVINGSAID POLYSULFIDE AND SAID LOSSEDEN SCALE FROM SAID SURFACE.